Discharge lamps for general service illumination require a long lifetime of at least 6000 hours, coupled with good color rendition. Thus, such lamps must have a high Ra index and, for overall color rendition, a minimum value of Ra.sub.8 =80 is desirable. The red index, R9, which provides a measure of color rendition within the red spectral range is of particular importance. Heretofore it was not possible to provide lamps which have a satisfactory color rendition coupled with long lifetime.
The referenced Dobrusskin U.S. Pat. No. 3,842,307 describes a metal halide lamp which has a special fill designed to provide NDL, that is, neutral white light color. The fill includes sodium halides and various rare -earth halide additives. This fill, however, is not suitable for warm white light, that is, for WDL light, since the wall loading to obtain such a light color would be excessively high. Such high wall loading, in combination with the fact that the major portion of the rare-earth halides in the lamp are present as condensates, substantially decreases the lifetime. Condensates within the lamp lead to devitrification of the light bulb, due to chemical reaction of the fill substances with the quartz glass of the bulb.
It has been proposed to utilize a fill for a lamp which provides WDL light, including a sodium tin fill--see the referenced U.S. Pat. No. 4,171,498. This lamp, however, does not provide satisfactory color rendition and the incomplete removal of undesired residual gases has led to excessive corrosion of the electrodes.
A pure sodium-scandium fill as proposed in U.S. Pat. No. 4,890,030 has long lifetime, that is, an operating rated life of over 6000 hours. The color rendition characteristics, however, are poor. This lamp has been used widely in the U.S. due to its long lifetime in spite of the overall color rendition index Ra.sub.8 of only about 70 and an R9 of-90.
Various additives-to the sodium-scandium system have been tested, particularly halides of thorium (see European 0 220 633, Gilliard et al) and of thallium. U.S. Pat. No. 4,866,342, Ramaiah et al, describes a lamp providing color temperatures between 3800 and 4600K., corresponding to the NDL light color, and designed for high power, that is, 400 W and over. This lamp uses a fill with a molar relationship of sodium halide: scandium halide of 25:1 to 50:1; and a proportion of sodium halide to thallium halide of 75:1 to 280:1.
U.S. Pat. No. 4,594,529 recommends similar relationships for use in an automotive discharge lamp which, typically, has a color temperature of about 4500K., that is, neutral white. The color rendition, actually, is not important in this type of application.
It has been tried to improve the sodium-scandium system, for example by the addition of elementary scandium, see U.S. Pat. No. 4,709,184, or coating of the discharge vessel, see European 0 220 633 and the U.S. Pat. No. 4,709,184; alternatively, evacuating of an outer envelope or vessel and addition of a heat retention or heat damming tube has been suggested, see U.S. Pat. No. 4,890,030.
The European Specification 0 215 524, Meulemans et al, is directed to obtaining a lamp having reasonably good color rendition and low color temperature, corresponding to the WDL or warm white color standard. This lamp is based on a sodium-thallium system with, possibly, addition of rare earths, including scandium. The wall loading of such a lamp, however, is very high, for example equal to or over 25 W/cm.sup.2, and typically 60 W/cm.sup.2. This high wall loading requires a ceramic discharge vessel. It is necessary to maintain specific geometrical relationships with respect to the discharge vessel volume and the electrode arrangement. While this patent describes a theoretically interesting solution, it is undesirable from a practical point of view since it requires ceramic material involving an entirely new technology of lamp manufacture, thus substantially increasing the costs of the lamp, and introducing additional manufacturing problems. These problems relate, especially, to long-term sealing and tightness of the connecting leads to the electrodes and accessory elements as well as the development of glass melts or glass solders which are resistant to halides.
It has not been possible, heretofore, to find a satisfactory solution to-the development of a gas discharge lamp of low power which, at the same time, has long lifetime as well as good color rendition, and thus is suitable for general-service use, such as interior illumination of living areas and stores, offices and the like. The problem appears to be that the desired characteristics of the lamp are, technically, mutually exclusive and that, further, light output deteriorates as the color rendition index becomes better and increases.